NAN-190: agonist and antagonist interactions with brain 5-HT1A receptors.
NAN-190 has been reported to be a 5-HT1A antagonist in drug discrimination studies. In order to determine if the effect of NAN-190 was directly due to competitive inhibition at 5-HT1A receptors, 5-HT1A-mediated inhibition of adenylyl cyclase in hippocampal membranes was investigated. NAN-190 (10(-10)-10(-5) M), by itself, was found to have no effect on forskolin-stimulated adenylyl cyclase. NAN-190, however, did shift the 5-carboxamidotryptamine (a 5-HT1A agonist) log-concentration inhibition curve to the right in a concentration-dependent manner, typical of competitive antagonism. Schild analysis revealed a KB of 1.9 nM for NAN-190. Thus, NAN-190 appeared to be a potent competitive 5-HT1A antagonist using the in vitro adenylyl cyclase system. [3H]NAN-190 was synthesized and its 5-HT1A receptor binding properties were characterized and compared with the 5-HT1A agonist radioligand, [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT). The 5-HT1A agonists, serotonin (5-HT) and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) competed with equal affinities regardless of the radioligand used to label the 5-HT1A receptors. [3H]NAN-190 and [3H]8-OH-DPAT labeled the same number of sites in rat hippocampus, striatum and frontal cortex. Guanosine-5'-O-(3-thio)triphosphate (GTP gamma S) and 5-guanylyl-imidodiphosphate (GppNHp), non-hydrolyzable analogs of GTP, inhibited specific [3H]NAN-190 binding. Adenosine-5'-O-(3-thio)triphosphate (ATP gamma S) and 5-adenylyl-imidodiphosphate (AppNHp) were ineffective. This guanylyl nucleotide-specific effect is generally associated with agonist radioligand binding to a GTP-binding protein coupled receptor. However, [3H]8-OH-DPAT was far more sensitive than [3H]NAN-190 to the Bmax reducing effects of GTP and GTp gamma S. We propose that the test for a reduction in Bmax by non-hydrolyzable guanylyl nucleotides may be more sensitive than other tests for quantifying agonist activity and may demonstrate that NAN-190 has low intrinsic activity. In summary, NAN-190 displayed antagonist-like properties in functional models of 5-HT1A receptor activity and possibly partial agonist-like properties in radioligand binding experiments.
Rydelek-Fitzgerald
L
L
Department of Pharmacology and Toxicology, Albany Medical College, NY 12208.
Teitler
M
M
Fletcher
P W
PW
Ismaiel
A M
AM
Glennon
R A
RA
D000262:Adenylyl Cyclases / Q000187:drug effects; D000266:Adenylyl Imidodiphosphate / Q000494:pharmacology; D000818:Animals; D001921:Brain / Q000187:drug effects* / Q000378:metabolism; D005576:Colforsin / Q000494:pharmacology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006160:Guanosine Triphosphate / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008297:Male; D008566:Membranes / Q000378:metabolism; D010879:Piperazines / Q000494:pharmacology*; D011869:Radioligand Assay; D051381:Rats; D011919:Rats, Inbred Strains; D011985:Receptors, Serotonin / Q000187:drug effects*; D012702:Serotonin Antagonists / Q000494:pharmacology*
D010879:Piperazines; D011985:Receptors, Serotonin; D012702:Serotonin Antagonists; C058895:1-(2-methoxyphenyl)-4-(4-(2-phthalimido)butyl)piperazine; D005576:Colforsin; D000266:Adenylyl Imidodiphosphate; D006165:Guanylyl Imidodiphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D000262:Adenylyl Cyclases